A spread spectrum communication system is such a system that transmission bandwidth is substantially higher than data rate. In a direct sequence spread spectrum communication system, at transmitter side, a data sequence is modulated by a pseudo random chip sequence to generate spread spectrum signal. Usually the chip rate of the pseudo random sequence is much higher than the rate of the data sequence, resulting in the spread spectrum signal having significant wider bandwidth than that of the data sequence itself. The spread spectrum signal is then transmitted over a communication media to a receiver. At the receiver side, the received spread spectrum signal is multiplied by a local pseudo random chip sequence to despread and recover the contained information.
Having many advantages over other communication systems, direct sequence spread spectrum communication system is one of the major communication systems widely used in today's society. However, regular direct sequence spread spectrum communication system has some disadvantages. One of the major disadvantages is low spectrum efficiency. Nowadays more and more applications require higher and higher data rate but the available bandwidth is both very expensive and limited. The low spectrum efficiency in a regular direct sequence spread spectrum communication system will greatly restrict its opportunity to be used in many high data rate applications.
Multi-channel direct sequence spread spectrum communications system is one of the attempts to increase data rate and therefore to improve the spectrum efficiency of direct sequence spread spectrum communication system. Basically, a multi-channel direct sequence spread spectrum communication system transmits a set of spreading signals simultaneously over a given frequency bandwidth.
Beside high spectrum efficiency, a communication system is also expected to be able to provide different data rates to meet various transmission requirements, which are determined by various applications and by different transmission environments.
In today's society, there are many different transmission applications such as transmitting voice, sending short text message, making video conference call, downloading files from internet, emailing photos to friend and watching movie. Usually different application requires different transmission rates. A good communication system should be able to provide many different transmission rates to meet the requirements of different applications.
Different transmission environments also require a communication system, especially a wireless communication system, to have different data rates. When transmission condition is good, such as a mobile station is close to a base station, it may be desirable for a communication system to transmit message with higher data rate. When transmission condition deteriorates, such as a mobile station is far away from a base station, or there is a serious fading, it may be attractive for a communication system to transmit message with lower data rate in order to have reliable communication.
A multi-channel communication system has the inborn capability to support many different data rates. To have different rates, a multi-channel communication system can simply use part of its channels for data transmission and prevent the rest channels from transmitting. For example, if only one channel is used for data transmission and the rest channels are shut off, then one data rate can be provided. If two channels are used for data transmission and the rest channels are shut off, then another data rate can be provided. If symbol repetitions are allowed, more data rates can be provided. The combinations of different number of active channels and different number of symbol repetitions can result in many different data rates.
Usually, a receiver is informed the change of data rate by a transmitter through upper layer of protocol, which not only costs the capacity of a communication system, but also cause overall delay. In a packet-switched communication system, the packet received could come from total different source than the previous one and therefore generally there is no any relation between two adjacent packets. When data rate is high and transmission environment changes rapidly, the delay could cause a receiver of a packet-switched communication system having no enough time to decode the intended data rate and therefore unable to correctly demodulate.
Therefore, there is a need to provide a mechanics for a receiver in a multi-channel direct sequence spread spectrum communication system to automatically detect the data rate set up by a transmitter.